1. Technical Field
The present invention relates to a wireless communication apparatus capable of obtaining higher receiver sensitivities even when transmission signals are mixed into receiving circuits.
2. Background Art
Very recently, instead of bar codes, a specific attention has been paid to non-contact type wireless tags known as RFID (Radio Frequency Identification) tags and the like. Then, it is predictable that these RFID tags and the like are attached to any sorts of articles, and these articles provided with these RFID tags are supplied in markets. Under such a circumstance, wireless communication apparatuses such as reader/writer apparatuses have been popularized which communicate with these RFID tags in non-contact manners.
Communication systems operable between these RFID tags and wireless communication apparatuses may be mainly classified into the below-mentioned communication systems. A first type is a communication system based upon electromagnetic induction. Within this electromagnetic induction type communication system, two sorts thereof have been proposed: That is, in one sort of the electromagnetic induction type communication system, a usable frequency is selected to be 400 KHz to 530 KHz; an electronic component (coils) having a high electromagnetic coupling degree such as a transformer is employed; and a communication is established between a coil of an RFID tag and a coil of a reader by utilizing mutual induction and mutual electromagnetic induction. In the other sort of the electromagnetic induction type communication system, electromagnetic waves having frequencies lower than or equal to 250 KHz, or a frequency of 13.56 MHz are utilized; an electronic component (coils) having a relatively low electromagnetic coupling degree is employed; and a communication is established by utilizing induction voltages between these coils. A communication distance of the former type system is short, namely several centimeters at maximum, whereas a communication distance of the latter type system is relatively long, namely several tens of centimeters.
In contrast to the above-described electromagnetic induction type communication systems, electric wave systems have been proposed as the second type. That is, microwaves radiated from an antenna of a wireless communication apparatus are received by a plane-wave antenna of an RFID tag, and data is transmitted by utilizing reflection waves from this RFID tag. Similarly, within the above-described microwave system, there are tow sorts of communication systems, namely, data is communicated by utilizing microwaves having a frequency of 2.45 GHz; and data is communicated by utilizing electric waves of the UHF (860 MHz to 960 MHz) frequency band. A communication distance of the former microwave system is approximately one meter at maximum, whereas a communication distance of the latter microwave system is relatively long, namely, eight meters at maximum.
There is one problem in such a case that a communication is carried out by employing either the electromagnetic induction system or the electric wave system. That is to say, when a single antenna is commonly used in both a transmission side and a reception side and the antenna receives a reflection wave from an RFID tag, a transmission wave generated on the transmission side is wrapped into a reflection wave and is mixed with the reflection wave, which constitutes an interference signal. As a result, the interference signal may give an adverse influence to a reception signal derived from the RFID tag. Also, even in such a case that antennas are independently employed in transmission and reception operations without employing an antenna duplexer, a transmission wave supplied from the transmission side is directly received, or is reflected on a peripheral article and then is received by the reception antenna provided on the reception side, so that this transmission signal may interfere with the reception signal, which may give an adverse influence to the reception signal.
As a consequence, conventionally, the following methods have been proposed (refer to, for instance, JP-A-H8-122429): That is, while offset waves (compensating signals) are produced in order to offset (compensate) unwanted waves which are mixed and interfere with reception systems when transmission waves are transmitted, the produced offset waves are synthesized with the unwanted waves.
In the method as described in JP-A-H8-122429, a phase and an amplitude of such a signal which has been demultiplexed from the transmission system are controlled by a variable phase shifter and a variable attenuator so as to produce an offset wave, and then, this offset wave is synthesized with a reception wave by a multiplexer in order to cancel the unwanted wave. The setting operation of the offset wave is controlled by manually operating both the variable phase shifter and the variable attenuator in such a manner that a transmission signal is transmitted from a wireless communication apparatus (interrogator) under such a condition that no reflection response is transmitted from an RFID tag (responder), and a level of a synthesized signal between a signal mixed into a reception system and the offset signal may become a minimum level under this condition.
However, in this system, it is not clear and concrete how the level of the synthesized signal is adjusted to a minimum level. Moreover, for instance, in such a case that a metal article and the like are present which may give an adverse influence to a wireless communication, and positions of the metal article are moved, a large adverse influence may be given to the synthesized signal. Then, this conventional system cannot produce offset waves (compensating signals) in response to circumferential changes in a real time manner. As a result, other wireless communication apparatuses have been proposed (refer to, for instance, JP-A-2006-14072). That is, the wireless communication apparatuses can perform offset operations by utilizing offset waves in response to such an environmental change in a real time manner, and can maintain higher receiver sensitivities.
The wireless communication apparatus as described in JP-A-2006-14072 includes a transmitting unit, a receiving unit, a cancel circuit, an RSSI (Received Signal Strength Indictor) circuit, and a control circuit. The transmitting unit is capable of transmitting a carrier wave to access a wireless tag to the wireless tag. The receiving unit is capable of receiving a transmission signal sent from the wireless tag in response to the transmission signal transmitted from the transmitting unit. The cancel circuit generates an offset wave for offsetting an unwanted wave which may be generated based upon the transmission signal from the transmitting unit when receiving the transmission signal. The RSSI circuit detects a strength of a reception signal received by the receiving unit, which has been offset by the offset wave. The control circuit outputs a carrier wave so that the transmitting unit transmits the carrier wave before the modulation wave is outputted from the transmitting unit so that the modulated wave is transmitted to the wireless tag, and change a phase and an amplitude of the offset wave generated by the cancel circuit in response to a detection result obtained in the RSSI circuit to control the respective cancel circuit and other units in order to set an optimum value.
As previously described, in the wireless communication apparatus described in JP-A-H8-122429, the transmission signal is transmitted from the wireless communication apparatus; and the offset wave is controlled by manually operating the variable phase shifter and the variable attenuator in such a manner that the level of the synthesized signal between the signal mixed in the reception system and the offset wave may become minimum.
However, in this conventional system, although the variable phase shifter and the variable attenuator are manually controlled, the concrete control operations must be carried out by a user; there is no clear instruction how to control these variable phase shifter and variable attenuator; and further, an external influence may be readily given to these control operations. In addition, this conventional system cannot produce the offset waves (compensation signals) in response to the circumferential changes in the real time mode.
As to this technical difficult point, the wireless communication apparatus described in JP-A-2006-14072 can perform the offset operation by employing the offset waves in response to the environmental changes in the real time mode, and thus, can maintain the higher receiver sensitivities. To this end, the wireless communication apparatus is equipped with the RSSI circuit for detecting the strength of the reception signal received by the receiving unit, which has been offset by the offset wave; and the control circuit for controlling the respective cancel circuit and other relevant units in such a manner that the phase and the amplitude of the offset wave generated by the cancel circuit are changed in response to the detection result obtained in the RSSI circuit so as to set the optimum value.
However, in the wireless communication apparatus described in JP-A-2006-14072, while a large number of monitoring points are set, the received signal strengths are sequentially measured at the respective monitoring points, and then, such a monitoring point that the received signal strength becomes minimum is defined as an optimum point. However, the above-described measuring method can hardly become an effective measuring method. Moreover, there is a limitation as to the detection precision of the RSSI circuit. Also, while both a phase and an amplitude of a cancel signal are changed based upon an output signal supplied from a VCO (Voltage-Controlled Oscillator) in response to a detection result obtained from the RSSI circuit, the noise from the transmission signal after the baseband signal has been mixed with the detection signal cannot be removed. As a result, if the RSSI circuit is employed, then a lengthy time is required for canceling the unwanted wave, and further, there is a limitation even in the canceling level for the unwanted wave.
In addition, although the above-described wireless communication apparatus is equipped with an I/Q modulating and demodulating circuit for demodulating the reception signal, the RSSI circuit is provided so as to change the phase and the amplitude of the offset wave, which is independent from the I/Q modulating and demodulating circuit. As a result, there is such a risk that the basic arrangement inherent to the wireless communication apparatus cannot be sufficiently utilized.